These days, compact, slim imaging devices are incorporated in compact, slim electronic appliances called portable terminals (for example, cellular phones, PDAs (personal digital assistants), etc.). Between such a portable terminal and, for example, an electronic appliance at a remote location, information such as sound information and image information is transferred bidirectionally.
Image sensors used in imaging devices are, for example, solid-state image sensors such as CCD (charge-coupled device) image sensors and CMOS (complementary metal-oxide semiconductor) image sensors. Nowadays, as imaging lenses for forming a subject image on those image sensors, lenses made of resin are used that can be mass-produced inexpensively and that thus contribute to cost reduction.
As such imaging lenses, in particular as imaging lenses used in imaging devices (so-called camera modules) incorporated in portable terminals, there are widely known those comprising three plastic lenses and those comprising one glass lens and two plastic lenses. Inconveniently, however, these imaging lenses do not promise easy compatibility between further size reduction and higher mass-producibility due to technical limitations.
As one measure to overcome this inconvenience, Patent Document 1 listed below deals with the replica method. The replica method is a method for forming a large number of lenses (lens elements) on a single lens substrate (wafer). A lens substrate (lens unit) including a plurality of lenses formed by this method arranged in an array is first joined to image sensors in the shape of a wafer (sensor wafer) and is then split. In the lens unit so split, the imaging lens corresponding to an imaging sensor is referred to as a wafer scale lens (cemented compound lens), and the module including the wafer scale lens and the imaging sensor is referred to as a wafer scale camera module.
And Patent Document 1 discloses an imaging lens including a wafer scale lens (an optical element having a lens contiguous with at least one substrate surface of the lens substrate; also referred to as a lens block) formed by the replica method. Incidentally, in the wafer scale lens in this imaging lens, a diffractive surface and a refractive surface are formed simultaneously on a lens substrate, so that with those surfaces the imaging lens corrects chromatic aberration.
Patent Document 1: JP-A-2006-323365